This invention generally relates to micromachined ultrasonic transducers. In particular, the invention relates to capacitive micromachined ultrasonic transducers (cMUTs). One specific application for cMUTs is in medical diagnostic ultrasound imaging systems.
Conventional ultrasound imaging transducers generate acoustic energy via a piezoelectric effect in which electrical energy is converted into acoustic energy using a poled piezoelectric ceramic material. The acoustic energy that is transmitted in the forward direction, which is in the direction of the patient being scanned, is coupled to the patient through one or more acoustic matching layers. However, the acoustic energy transmitted in the direction away from the patient being scanned is typically absorbed in and/or scattered in an acoustic backing material located on the backside of the transducer array. This prevents the acoustic energy from being reflected from structures or interfaces behind the transducer and back into the piezoelectric material, thereby reducing the quality of the acoustic image obtained from reflection within the patient. Numerous compositions for the acoustic backing material are known. For example, the acoustic backing material may consist of a composite of metal particles (e.g., tungsten) in an attenuating soft material such as rubber, epoxy or plastic. Other acoustic backing material compositions may also be used.
The ultrasonic transducers used for medical diagnostic imaging have broad bandwidth and high sensitivity to low-level ultrasonic signals, which characteristics enable the production of high-quality images. Piezoelectric materials that satisfy these criteria and have been conventionally used to make ultrasonic transducers include lead zirconate titanate (PZT) ceramic and polyvinylidene fluoride. However, PZT transducers require ceramic manufacturing processes that are different from the processing technologies used to manufacture other parts of an ultrasound system, such as semiconductor components. It is desirable that ultrasonic transducers be manufactured using the same processes used to fabricate the semiconductor components.
Recently semiconductor processes have been used to manufacture ultrasonic transducers of a type known as micromachined ultrasonic transducers (MUTs), which may be of the capacitive (cMUT) or piezoelectric (pMUT) variety. cMUTs are tiny diaphragm-like devices with electrodes that convert the sound vibration of a received ultrasound signal into a modulated capacitance. For transmission the capacitive charge is modulated to vibrate the diaphragm of the device and thereby transmit a sound wave.
One advantage of MUTs is that they can be made using semiconductor fabrication processes, such as microfabrication processes grouped under the heading “micromachining”. As explained in U.S. Pat. No. 6,359,367:                Micromachining is the formation of microscopic structures using a combination or subset of (A) Patterning tools (generally lithography such as projection-aligners or wafer-steppers), and (B) Deposition tools such as PVD (physical vapor deposition), CVD (chemical vapor deposition), LPCVD (low-pressure chemical vapor deposition), PECVD (plasma chemical vapor deposition), and (C) Etching tools such as wet-chemical etching, plasma-etching, ion-milling, sputter-etching or laser-etching. Micromachining is typically performed on substrates or wafers made of silicon, glass, sapphire or ceramic. Such substrates or wafers are generally very flat and smooth and have lateral dimensions in inches. They are usually processed as groups in cassettes as they travel from process tool to process tool. Each substrate can advantageously (but not necessarily) incorporate numerous copies of the product. There are two generic types of micromachining . . . 1) Bulk micromachining wherein the wafer or substrate has large portions of its thickness sculptured, and 2) Surface micromachining wherein the sculpturing is generally limited to the surface, and particularly to thin deposited films on the surface. The micromachining definition used herein includes the use of conventional or known micromachinable materials including silicon, sapphire, glass materials of all types, polymers (such as polyimide), polysilicon, silicon nitride, silicon oxynitride, thin film metals such as aluminum alloys, copper alloys and tungsten, spin-on-glasses (SOGs), implantable or diffused dopants and grown films such as silicon oxides and nitrides.The same definition of micromachining is adopted herein.        
Acoustic energy generated using a capacitive micromachined ultrasonic transducer device does not rely on a piezoelectric material to generate ultrasonic energy. Rather, the basic structure of a cMUT is that of a conductive membrane or diaphragm suspended above a conductive electrode by a small gap. When a voltage is applied between the membrane and the electrode, Coulombic forces attract the membrane to the electrode. If the applied voltage varies in time, so too will the membrane position, generating acoustic energy that radiates from the face of the device as the membrane moves in position. While the acoustic energy is generated primarily in the forward, or patient, direction, some fraction of the acoustic energy will be propagated into the cMUT supporting structure. This structure is commonly a silicon wafer. An acoustic backing material is therefore needed to prevent reflection of this energy from the silicon/air interface at the back surface of the wafer back into the cMUT device. This is equally true for pMUT devices.
U.S. Patent Application Pub. No. US 2002/0048219 discloses a microfabricated acoustic transducer with suppressed substrate modes. This publication discloses the application of acoustic damping material on the backside of the transducer substrate.
A MUT device benefits from a backing material in more ways than simply acoustic performance. A MUT device manufactured on a silicon wafer is fragile and requires great care during the manufacturing process to not become damaged. For example, the electrical connection to the MUT device may be made by lamination of a flexible circuit. During this process, pressure is applied to the silicon wafer to bond electrical connections. Unevenly applied pressure can lead to fracture of the device. The thinned substrate onto which the MUT is built is very fragile and could benefit from a supportive structure.
There is a need for structures and methods for lending additional support to MUT (cMUT and pMUT) devices, while also enhancing the ability to damp undesirable acoustic waves that exit the rear face of the substrate of the MUT device.